This invention relates to an improvement on the sliding sheath latch mechanism of the general type shown in U.S. Pat. No. 5,057,086 granted Oct. 15, 1991, to John A. B. Dillard and James A. Orr.
In that patent there is disclosed a sliding sheath that automatically covers the needle of a syringe when the operator finishes use. A latch mechanism maintains the sheath in its needle-covering position so that a person cannot accidentally prick himself or another person with the newly contaminated needle. The syringe sheath is propelled to its needle-covering position by a spring, preferably helical, carried on the exterior of the syringe body. During use of the syringe the operator manually pulls back the sheath to expose the needle by compressing the spring. When the operator has completed use of the syringe, he manually releases the sheath and the spring propels the sheath forwardly. The latch mechanism is activated by the same expansion of the spring to latch the sheath in its needle-covering position. Normally, the operator completely releases the sheath, and the spring slams the latch to the needle-covering position.
A problem sometimes arises, however, when the operator gently allows the spring to expand to extend the sheath over the needle. The last stages of spring expansion under this condition has such little force left that it sometimes does not actuate the latch, and the sheath can then slide under the impact of a blow, exposing the needle tip.